Optical frequency conversion involves generating an output optical signal that is a frequency-adjusted replica of an input optical signal. Optical frequency conversion systems are used in a wide range of applications, including environmental sensing, optical communication, and laser medicine. Optical frequency conversion systems are particularly useful in all-optical fiber-based communication networks. These networks typically include hierarchies of linked local sub-networks and long haul routes, at least some of which may operate at the same optical frequency. The use of wavelength division multiplexing (WDM) in such networks can increase signal capacity and enhance network flexibility. Optical frequency conversion systems are used in WDM optical communications networks to shift frequencies for routing to sub-network links, to perform phase conjugation operations that are used over long haul routes, and to perform various types of optical processing functions.
Many different optical frequency conversion techniques have been proposed. In some approaches, an electrical signal is used to modulate the frequency of an input optical signal based on the electro-optic effect or the acousto-optic effect. These electronic modulation techniques, however, are limited by the speed of the modulation control electronics. Several all-optical frequency conversion techniques also have been proposed, including cross-gain modulation, cross-phase modulation, and four-wave-mixing frequency conversion techniques.
In one non-degenerate four-wave-mixing approach, an input optical signal and a pump optical signal are mixed in a nonlinear medium (e.g., a medium exhibiting a nonlinear index of refraction), which generates conjugate signal frequencies as a function of the input signal frequencies. An optical filter selectively passes an output optical signal corresponding to a frequency-shifted replica of the input optical signal.
In another approach, an input optical signal is propagated through an electro-optic material having a ⅓ rotational symmetry. The input optical signal is circularly polarized in the ⅓ rotational symmetry direction of the electro-optic material. A rotating electric field is applied in a direction perpendicular to the propagation direction of the input optical signal to shift the frequency of the input optical signal. The amount by which the input optical signal frequency is shifted may be increased by providing a feedback loop with a shutoff switch and cycling the input optical signal through the electro-optic material multiple times. The amount of frequency conversion is proportional to the number of times the input optical signal is cycled through the electro-optic material.
In a parallel phase modulation optical frequency conversion approach, an input optical signal is divided into multiple sub-signal light components with prescribed relative phase relationships. The sub-signal light components are distributed into respective optical phase modulators. Each optical phase modulator is driven by a respective radio frequency (RF) signal having a prescribed relative phase relationship with the sub-signal light component passing through the corresponding optical phase modulator. The signals transmitted from the optical phase modulators are combined to produce an output optical signal corresponding to the modulation product of the optical input signal and the RF drive signals, except with a low-order modulation product canceled out and suppressed.
Another optical frequency conversion approach induces a phase shift using moving reflectors, which may be moving mirrors or an acousto-optical filter. The moving reflectors oscillate (i.e., they move first in a first direction and then in a second direction). Two different reflectors are used so that the light can be switched between the reflectors. During a first portion of the cycle the light is coupled to the first reflector which moves in the first direction. The second reflector is out of phase with the first reflector, and the light is switched to that second reflector during a second portion of the cycle. The second reflector also is moving in the first direction when the light is applied thereto. In this way, the light obtains a constant direction Doppler shift.